The present invention relates to a process for the production of aluminum by carbothermic reduction of alumina and to a reactor for the production of aluminum by carbothermic reduction of alumina.
The direct carbothermic reduction of alumina has been described in U.S. Pat. No. 2,974,032 (Grunert et al.) and it has long been recognized that the overall reaction: Al2O3+3C=2Al+3CO (1) takes place, or can be made to take place, in two steps: Al2O3+9C=Al4C3+6CO (2); and Al4C 3+Al2O3=6Al+3CO (3).
Reaction (2) takes place at temperatures below 2000xc2x0 C. Reaction (3), which is the aluminum producing reaction, takes place at appreciably higher temperatures of 2200xc2x0 C. and above; the reaction rate increases with increasing temperature. In addition to the species stated in reactions (2) and (3), volatile species including gaseous Al, gaseous aluminum suboxide (Al2O) and CO are formed in reactions (2) and (3) and are carried away with the off gas. Unless recovered, these volatile species will represent a loss in the yield of aluminum. Both reactions (2) and (3) are endothermic.
U.S. Pat. No. 6,440,193 relates to such a process for carbothermic production of aluminum where aluminum carbide is produced together with molten aluminum oxide in a low temperature compartment. The molten bath of aluminum carbide and aluminum oxide flows from the low temperature compartment into a high temperature compartment where the aluminum carbide (Al4C3) is reacted with the aluminum oxide (Al2O3) to produce aluminum. In the high temperature compartment, aluminum forms a layer on top of a molten slag layer and is tapped from the high temperature compartment. The off-gases from the low temperature compartment and from the high temperature compartment, which contain Al vapor and volatile aluminum suboxide (Al2O) are reacted to form Al4C3. The low temperature compartment and the high temperature compartment are located in a common reaction vessel, with the low temperature compartment being separated from the high temperature compartment by an underflow partition wall. The molten bath containing aluminum carbide and aluminum oxide produced in the low temperature compartment continuously flows under the partition wall and into the high temperature compartment by means of gravity flow which is regulated by tapping of aluminum from the high temperature compartment. The energy needed to maintain the temperature in the low temperature compartment and in the high temperature compartment is provided by separate energy supply systems.
In the second step, reaction (3), excess carbon is necessary to promote the production of aluminum. In order to maintain a sufficient carbon content in the high temperature compartment, it is necessary to add additional carbon to the high temperature compartment. According to U.S. Pat. No. 6,440,193 the additional carbon is added through a supply means arranged in the roof of the high temperature compartment thereby requiring the additional carbon to pass through the top layer of molten aluminum in the high temperature compartment and into the molten bath in the high temperature compartment.
It has been discovered that the addition of carbon material to the top of the molten aluminum can cause a reverse reaction of the aluminum as well as poor distribution of the carbon in the high temperature reaction zone. In order to overcome this problem, it has been discovered that the additional carbon material should be added directly into the slag layer and below the upper aluminum layer, thereby keeping the composition of the slag layer more uniform during the formation of aluminum in the high temperature compartment. It has been further discovered that the additional carbon material should be distributed as evenly as possible in the slag layer in the high temperature compartment. Finally, it has been discovered that the additional carbon material should be added in a controllable manner.
In order to take advantage of these discoveries, a process and a reactor have been invented. Specifically, the process of the present invention comprises adding additional carbon material to the slag as it flows below the partition wall from the low temperature compartment to the high temperature compartment. The reactor of the present invention comprises a means for supplying the additional carbon material to the slag as it flows below the partition wall from the low temperature compartment to the high temperature compartment.
In a preferred embodiment of the present invention, the means for supplying the additional carbon material to the slag layer is an opening in the lower portion of the partition wall. More specifically, the partition wall is hollow with an opening in the bottom that allows additional carbon material to flow out the bottom of the partition wall and into the underflow of slag as it moves from the low temperature compartment to the high temperature compartment of the reactor. A transport means, such as a screw or ram or a combination of a screw and a ram, is employed to move the additional carbon through the wall. Preferably, the hollow partition wall is vertically movable so as to vary the height of the opening in the slag underflow.
By adding the additional carbon material to the underflow of slag at the partition wall, the additional carbon material is added directly into the slag, below the level of the upper aluminum layer, and the amount of added carbon material can be evenly distributed throughout the slag in the high temperature compartment. Since the partition wall is vertically movable, the point of addition for the additional carbon material can be varied. Normally the vertical position of the wall is only adjusted when the furnace is not in operation. Furthermore, the amount of carbon added to the slag can be controlled by the speed at which the transport means moves the additional carbon material through the wall.
Preferably, the hollow area and the opening in the partition wall extend across the entire wall. Alternatively, the hollow area can be divided into a series of channels or into vertically oriented conduits. Each conduit has an opening at the base of the wall to conduct additional carbon material downward and feed the additional carbon material into the underflow of slag.
Broadly, the present invention is a process for supplying additional carbon material to a reactor for carbothermic production of aluminum wherein the reactor is divided into a low temperature compartment and a high temperature compartment by a hollow underflow partition wall. A molten bath or slag comprising aluminum carbide and aluminum oxide is produced in the low temperature compartment. The molten bath of aluminum carbide and aluminum oxide flows under the hollow underflow partition wall into the high temperature compartment where the aluminum carbide is reacted with alumina to produce aluminum which forms a layer on top of the molten slag bottom layer and where aluminum is tapped from the high temperature compartment. The additional carbon material is supplied to the molten bath of aluminum carbide and aluminum oxide through at least one opening in the hollow underflow partition wall, said opening being at a level below the layer of molten aluminum in the high temperature compartment. In other words, the opening is positioned in the wall at the level of the slag as it flows under the wall.
The reactor of the present invention is a reactor for carbothermic production of aluminum which comprises a reaction vessel comprising a low temperature reaction compartment and a high temperature reaction compartment. The low temperature compartment has means for supply of materials to said compartment and one or more electrodes for supplying electric operating current to said compartment, said electrode or electrodes being positioned for submersion in a molten bath which is produced in the low temperature compartment. The high temperature reaction compartment is separated from the low temperature compartment by means of a hollow partition wall. The hollow partition wall has at least one opening into the underflow of the molten bath which allows underflow of the molten bath from the low temperature reaction compartment to the high temperature compartment. A plurality of pairs of substantially horizontally arranged electrodes are arranged in the sidewall of the high temperature compartment of the reaction vessel for supply of electric current to said compartment. The high temperature compartment has an outlet for continuously tapping molten aluminum. The molten bath produced in the low temperature compartment flows into the high temperature compartment by gravity flow effected by tapping the top aluminum layer in the high temperature compartment. The at least one opening in the partition wall is positioned at a level below the layer of molten aluminum in the high temperature compartment.
In accordance with the present invention, the additional carbon material can take the form of coke, coal, agglomerated carbon powder or any other form. Also, additional carbon material can take the form of Al4C3, which is preferred in order to reduce the amount of CO gas produced in the high temperature compartment as well as to recycle Al4C3 from off-gas reactors connected to the high and low temperature compartments. Finally, Al4C3 filtered off from the produced aluminum tapped from the reactor can also be used as a form of additional carbon material.